Measurement of Near-Tip Field of Fast Propagating Cracks at Bifurcation

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(1) High-Speed Holographic MicroscopyReflection type high-speed holographic microscopy was successfully applied to take three successive microscopic photographs of fast propagating cracks at the instant of bifurcation.When a fast propagating crack of opening mode bifurcates into two cracks, three pulsed ruby lasers oscillate successively. The laser pulses illuminate the specimen and the crack is recorded as three holograms on a photographic plate by means of angle-multiplexing holography. Lens assisted method reconstructs the three crack images separately from the holograms. The spatial resolution is more than 180 lines/mm, and the frame interval was about 5 micro-seconds.(2) Attempted Branches and MicrocracksFrom the microscopic photographs taken by the high-speed holographic microscopy, one can know the behavior of small branched cracks just before and just after the main bifurcation.When a fast propagating crack bifurcates into two or more cracks, there appear several crack tips which propagate simultaneously in a small region about 1mm in diameter. Some of them stop propagating, and become attempted branches. However, some crack tips keep propagating, and they grow into a large bifurcation. Three dimensional structure of crack front edges or behavior of microcracks may play an important role for crack bifurcation.(3) Near-lip Field at BifurcationAt bifurcation, the stress field near a fast propagating crack tip deviates from the K-field which is theoretically predicted by the dynamic fracture mechanics. Such deviation occurs not only after bifurcation but also just before bifurcation. The deviation after bifurcation is due to the existence of several crack tips. On the other hand, the deviation just before bifurcation is thought to be due to the microcracks or attempted branches near the propagating crack tip. The deviation of the near tip field from the K-field may cause the large scale bifurcation.